It is often necessary or desirable to measure various compositional parameters of blood, such as the presence and/or concentration of blood constituents, such as blood gases, hydrogen ions (pH), electrolytes, glucose, red blood cells and the like. This can be accomplished in real time in an extracorporeal blood loop as described in Cooper U.S. Pat. No. 4,640,820, in vivo as disclosed in Grandparent U.S. Pat. No. 4,830,013, and in a sensor cassette as shown in parent application Ser. No. 229,617. The sensor cassette is located outside the body of the patient and may be attached to, for example, the arm of the patient. One important advantage of the sensor cassette is that only a relatively small amount of blood is needed to obtain accurate and reliable blood analysis.
The sensor cassette may include a housing having a wall defining a passage which is adapted to receive blood. A sensor is mounted on the housing in a position to be contacted by the compositional parameter of the blood in the passage. The sensor provides a signal in response to the compositional parameter of interest which is usable in determining the presence of, or a quantitative measure of, the compositional parameter of interest.